Police Commissioner Dermot Shea knows what he wants under his tree this Christmas: for New York to "get serious about guns." He’s not the only one with that wish. In a meeting Thursday with The Post’s Editorial Board, Shea pleaded for a tougher approach to getting illegal guns off the streets, whether through tweaks to current law or by bolstering the way the laws are carried out by prosecutors, judges, parole officers and the entire criminal-justice system.